Various concepts of electronic memory applications based on a phase change material (PCM) that has a different resistivity in its crystalline and amorphous states, and which can be electrically switched between these states, have been proposed. E.g., in U.S. Pat. No. 5,933,365, structures have been proposed where a phase change material is applied in a top-hole or a via-hole, i.e., conducting a current in the vertical direction, and contacting via one or more intermediate layers to metallic conductors (electrodes) above and below the phase change material via. Also, it is disclosed to apply pulse schemes to switch from the low resistivity state to the high resistivity state or vice versa. Also, a heating layer can be applied in series with the phase change material to better control the heating of the phase change volume and its environment.
An alternative structure is disclosed in WO04057618, where the phase change material is deposited as a line, i.e. in-plane, typically in a horizontal direction (in a plane parallel to that of the substrate), connecting to the electrodes at a distance away from the switching region of the PCM line. In these line concepts, the line of PCM is heated by sending a current through the (narrow) line. Depending on the amount of heating and the rate at which the PCM cools down, a part of the PCM within the line, called the switching zone, will transform from amorphous to crystalline or vice versa. As these phases are stable with time, the corresponding different resistance values of the crystalline and amorphous phases can be used for memory application. Measuring the resistance by sending a small readout current through the line does readout.
The above mentioned document also discloses a heater element that is connected electrically parallel to the phase change material, a.o., to be less susceptive to the difference in resistance of the PCM resistor during programming.
These memories can be used as (embedded) flash floating gate memories which are programmed and erased many times and keep their information even when the power supply is off, they can also be used as (embedded) SRAM and/or DRAM, as soon as its electrical, thermal and physical properties allow for it, i.e. when programming voltages and dissipated power are sufficiently low, and the endurance is sufficiently good to accommodate a large number of programming cycles while maintaining good performance. The line concept is especially attractive, as structures using this concept may be relatively easy to manufacture and can in principle be well scaled to small dimensions of memory element as a whole, and of the switching part of the phase change volume in particular.
WO2004/057618 discloses an embodiment of an electric device having a resistor a resistor comprising a phase change material being changeable or transformable between a first, e.g. crystalline, phase and a second, e.g. amorphous, phase within a switching zone, the resistor having a first electrical resistance when the phase change material is in the first phase and a second electrical resistance, different from the first electrical resistance, when the phase change material is in the second phase, the phase change material constituting a conductive path between a first contact area and a second contact area, a cross-section of the conductive path being smaller than the first contact area and the second contact area, resembling a so-called “dog bone” structure. The electric device further comprises a heating element able to conduct an electrical current for enabling a transition from the first phase to the second phase. The first phase and/or the second phase may be partially amorphous and partially crystalline. In the remainder the terms “crystalline” and “amorphous” are used to refer to a crystalline phase or a mainly crystalline phase, and to an amorphous phase or a mainly amorphous phase, respectively.
The resistor is electrically connected to a first conductor and a second conductor in such a way that its electrical resistance value can be measured. A transition from a crystalline phase with a relatively good electric conductivity to a amorphous phase with a relatively poor conductivity, requires sufficiently strong heating to melt the phase change material. When the heating ends, the phase change material cools down and when this occurs fast enough assumes a more amorphous phase.
When the heat input is sufficiently high to increase the temperature of the phase change material to above its crystallization temperature, when amorphous, a phase transition from the low conductivity phase to the high conductivity crystalline phase can occur.
The “dog bone” structure of the lines of memory material, as disclosed in WO2004/057618, serves to improve endurance. In the “dog bone” structure, the volume of phase change material in the line shaped part has a larger electrical resistance than the electrical contact resistance at the first and/or the second contact areas, independent of whether the phase change material is in the first or second phase. In such a device, Joule heating at the first contact area and/or at the second contact area is smaller than the Joule heating inside the volume of phase change material in the line shape part where the current density is high. This reduces the interaction between the phase change material and the other materials at the first and/or second contact areas, leading to an improved endurance compared to a structure without the “dog bone” structure. Moreover, the electrical power is dissipated, i.e. converted to heat, mainly at the position where the phase change occurs. By reducing the dissipation at positions where the phase change does not occur the total electrical power required for inducing a phase transition is reduced.
The heating can be achieved by passing an electric current through the phase change material, and/or through a heating element in thermal contact to the phase change material. In In WO2004/057618, the heating element is arranged to be electrically parallel with the resistor, such that the phase transitions in the phase change material are generated by its own resistive heating and the heat generated by the resistive heating of the heater. In U.S. Pat. No. 5,933,365 the heating layers are connected in series with the resistor (in fact they also constitute the contact between the phase change layer and the electrical contacts), and they heat the resistor by Joule heating. The heating layers can be thin-film structures deposited adjacent to the phase change material.
The heating can be achieved by passing an electric current through the phase change material, and/or through a heating element in thermal contact to the phase change material. In WO2005/057618, the heating element is arranged to be electrically parallel with the resistor, such that the phase transitions in the phase change material are generated by its own resistive heating and the heat generated by the resistive heating of the heater. In U.S. Pat. No. 5,933,365 the heating layers are connected in series with the resistor (in fact they also constitute the contact between the phase change layer and the electrical contacts), and they heat the resistor by Joule heating. The heating layers can be thin-film structures deposited adjacent to the phase change material.
The known electric device is an electrically writable and erasable memory cell, which carries information encrypted in the value of the electrical resistance. The memory cell is assigned, e.g. a “0” when the resistance is relatively low and a “1” when the resistance is relatively high. The resistance may be easily measured by supplying a voltage across the resistor and measuring the corresponding current. The memory element is written and erased by inducing a transition from a first phase to a second phase as described above.
It is observed that an electric device as described above deteriorates when repeatedly switched between the first phase and the second phase, i.e., the lifetime, also called life span or endurance, of the electric device is limited. The inventors have recognized that the origin of this lies in the physical structure and the thermal properties of the prior art structures.